A. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to an improved method for making a preform doped with a metal oxide from which such fibers can be produced.
B. Description of the Prior Art
As is known in the art, optical waveguide fibers consist of a higher index of refraction core surrounded by a lower index of refraction cladding. Depending on the type of fiber and its desired performance characteristics, the radial distribution of the index of refraction across the face of the fiber can be simple or complex. For example, single mode fibers typically have an index of refraction profile which is a simple step, i.e., a substantially uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding interface. On the other hand, to produce a high bandwidth multimode fiber requires achieving a nearly parabolic radial refractive index profile in the fiber core so as to minimize intermodal dispersion. See R. Olshansky, "Propagation in Glass Optical Waveguides", Reviews of Modern Physics, Vol. 51, No. 2, April, 1979, pages 341-367.
Optical waveguide fibers can be prepared by various techniques known in the art. The present invention is concerned with those techniques wherein a porous soot preform is formed and then consolidated. More particularly, the invention is concerned with vapor deposition soot laydown techniques for producing preforms.
Preforms produced by vapor deposition techniques typically are composed of silicon dioxide (SiO.sub.2) selectively doped with at least one metal or metalloid oxide (referred to generally herein as a "metal oxide") to provide the desired index of refraction profile. The preferred metal oxide dopant in commercial use today is germanium dioxide (GeO.sub.2), although other metal oxides, such as, titanium oxide, tantalum oxide, lanthanum oxide, antimony oxide, aluminum oxide, and the like, as well as mixtures of metal oxides, can be used as dopants. Since metal oxide dopants are one of the more expensive raw ingredients used in the preparation of optical waveguide fibers, it is important that the dopant be effectively incorporated in the preform with a minimum of loss.
In accordance with one vapor deposition technique, outside vapor deposition or "OVD", soot particles are formed by oxidizing and/or hydrolyzing halide materials, e.g., SiCl.sub.4 and GeCl.sub.4, in a burner. The preform is formed from the soot particles by moving the burner back and forth along the length of a rotating mandrel. See, for example, Bailey et al., U.S. Pat, No. 4,298,365. The distance between the mandrel and the burner is selected so that the soot particles collect on the mandrel in thin layers with each pass of the burner. The amount of halide materials supplied to the burner is adjusted during the soot laydown process so as to produce a dopant concentration in the preform which varies with radius. This dopant concentration profile is selected so that the finished fiber will have the desired index of refraction profile.
The burners used in soot laydown processes, such as the OVD process, have multiple orifices or outlet structures. The orifices carry the halide materials, the fuel for the burner, and oxygen for reaction with the fuel and the halide materials. Depending on the burner design and the specifics of the material being deposited, the various orifices can contain one or a mixture of these reactants. In addition, some of the orifices can carry inert gases, either alone or mixed with reactants, to serve as carriers or means for controlling the shape and temperature profile of the burner's flame. A typical burner design is shown in Moltzan, U.S. Pat. No. 3,698,936: a discussion of the temperature characteristics of the flame produced by such burners can be found in M. Elder and D. Powers, "Profiling of Optical Waveguide Flames", Technical Digest for the 1986 Conference on Optical Fiber Communication, Atlanta, Ga., page 74, 1986.
In the OVD process, once soot laydown has been completed, the mandrel is removed from the center of the preform, and the preform is mounted on a hollow handle. The preform is then ready for drying and consolidation in a consolidation oven. Drying and consolidation are accomplished by heating the porous preform to its sintering temperature and surrounding the preform with one or more drying gases, e.g., a mixture of helium and chlorine gas, and by passing such gases through the handle and down the center of the preform. Alternatively, drying gases can only be applied to the centerline of the preform. See, for example, Powers, U.S. Pat. No. 4,165,223. During the drying/consolidation process, once the preform pores are significantly closing, the flow of the drying gases may be stopped. The consolidation is performed sequentially over the length of the preform, with the tip of the preform being consolidated first and the portion of the preform near the handle being consolidated last.
Ideally, the consolidated preform should have uniform characteristics along its length. In practice, however, it has been found that the consolidation process results in "axial trends" along the length of the consolidated preform such that fiber produced from the tip of the preform has different properties from that produced from the middle of the preform, and fiber produced from the middle has different properties from that produced from the handle end.
These differences are plainly undesirable for numerous reasons. For example, the differences result in greater variability in the finished product. Moreover, if sufficiently large, the differences can result in unacceptable (rejected) material which does not meet the quality control standards for the product. This waste, in turn, results in higher production costs. In view of these and other problems, one of the primary goals of the present invention is to minimize the differences between fibers produced from different portions of the consolidated preform.
Some experimental studies of the behavior of metal oxides and, in particular, germanium dioxide (germania) during soot laydown have been performed. For example, Edahiro et al. have performed experiments which suggest that germania is deposited as a crystalline structure, not integrated with silica particles, when the temperature of the substrate upon which the deposition is occurring is below about 400.degree. C. On the other hand, when the temperature of the substrate is above about 500.degree. C., the germania is said to exist in a noncrystalline form dissolved in silica particles. See Edahiro et al., "Deposition Properties of High-Silica Particles in the Flame Hydrolysis Reaction for Optical Fiber Fabrication", Japanese Journal of Applied Physics, Vol 19, No. 11, November, 1980, pages 2047-2054. See also Kawachi et al., "Deposition Properties of SiO.sub.2 -GeO.sub.2 Particles in the Flame Hydrolysis Reaction for Optical Fiber Fabrication", Japanese Journal of Applied Physics, Vol. 19, No. 2, February 1980, pages L69-L71: and Optical Fiber Communications, vol. 1, 1985, Bell Telephone Laboratories, Inc., sections 3.3.2.3 and 3.3.2.4, pages 109-113.
Similarly, Sanada et al. have suggested that in the vapor axial deposition (VAD) soot laydown process, the germanium located in the central portion of the preform consists of glass particles composed of a solid solution of GeO.sub.2 and SiO.sub.2, while in the peripheral parts of the preform, a large percentage of the germania is in a hexagonal crystalline form. Sanada et al. ascribe these differences to differences in the temperature of the various parts of the preform as the deposition process takes place. See Sanada et al., "Behavior of GeO.sub.2 in Dehydration and Consolidation Processes of the VAD Method", Technical Digest for the 1984 Conference on Optical Fiber Communication, New Orleans, page 26, 1984. Sanada et al. have also stated that the presence of hexagonal GeO.sub.2 can affect the lengthwise fluctuation of refractive index profile during the dehydration of VAD preforms since this form of GeO.sub.2 is easily halogenated. Sanada's proposed solution to the problem is to adjust the dehydration process so that the hexagonal GeO.sub.2 is removed. See Sanada et al., "Behavior of GeO.sub.2 in Dehydration Process of VAD Method", Digest of 7th ECOC, Copenhagen, pages 2.1-1-2.1-4, 1981.
U.S. Pat. No. 4,627,866 and EPO Patent Publication No. 185,106 to Kanamori et al. are concerned with a VAD process in which fluorine is added in the soot laydown process. These references describe using higher oxygen partial pressures to aid in the addition of fluorine to a silica preform. The purpose of the increased oxygen partial pressures in these references is to thoroughly decompose fluorine-containing material (e.g. CCl.sub.2 F.sub.2, CF.sub.4, etc.) so that "further fluorine is effectively added" and "enough fine glass particles are synthesized." (U.S. Pat. No. 4,627,866, col. 2, lines 34-39). Significantly, the references contain no disclosure of the concept of providing oxygen to a burner inside of the burner's outermost fuel passageway in an amount greater than that which is stoichiometrically required to fully oxidize the fuel leaving the burner. In addition, the references do not disclose or suggest reducing the amount of undesired forms of a metal oxide which are generated during the creation of a porous glass preform and which can migrate along the length of the preform.
Although GeCl.sub.4 is mentioned in the Kanamori et al. references as a "gaseous glass raw material" for "synthesizing fine glass particles", there is no disclosure of any of the forms of the germanium/oxygen metal oxide or the relationship between the proportion of oxygen in the burner gas flows and the resulting forms of germanium/oxygen. These references are directed at the effect of oxidizing atmospheres on the deposition of fluorine-containing material, and they neither disclose nor suggest the use of such atmospheres to reduce axial trends in preforms by reducing the amount of undesirable forms of a metal oxide which tends to migrate during subsequent reheating. The only suggested use of such atmospheres in connection with the formation of germania requires the presence of a fluorine-containing material, which presence clearly impacts the effect of the oxidizing atmosphere.